1. Field of the Invention
The present invention relates to a projection eyepiece and method for aligning pattern areas on a substrate surface having a micro-optical device on an opposite surface side of the substrate. The projection eyepiece enables projection of a reticle image onto a first surface of a substrate, enabling receipt of a reflection of that reticle image from a micro-optical device located on a second and opposing surface of the substrate, and enabling comparison of the projected and received image to determine alignment of the point of incidence on the first surface with the micro-optical device on the second surface.
2. Description of the Related Art
Optical devices fabricated using photolithographic technology often require precise alignment of devices on both sides of a single substrate. For instance, it is sometimes necessary to etch optical lenslets, alignment marks, detectors or other devices into both sides of a thick (several millimeter) substrate, and to obtain a precise lateral arrangement of devices positioned on one side of the substrate with corresponding devices positioned on the opposite side of the substrate. Such precise alignment is difficult to achieve, particularly when the substrate is too thick for the mask aligner microscope or the substrate is opaque to visible light.
FIGS. 1A-1B illustrate how a conventional mask aligner (either visible or infrared) is used to align devices on opposite sides of a substrate, FIG. 1A showing the mask aligner focused on the distal (lower) substrate surface and FIG. 1B showing focus on the proximate (upper) substrate surface. More specifically, the microscope objective 11 of the mask aligner is positioned above the mask 12 and substrate 13. The mask pattern 15 is positioned on the lower surface of the mask and in contact with the photoresist coated on top of substrate 13. An alignment mark 14 has been previously etched into the lower surface of a substrate.
The mask aligner is designed to align an alignment mark 15 of mask 12 with the alignment mark 14 positioned on the lower opposing surface of substrate 13, so that the mask pattern can be transferred into the photoresist on the top surface of substrate 13. To achieve alignment, the microscope objective 11 of the mask aligner is alternatingly focused on the top and bottom alignment marks 14 and 15 by translating the microscope objective 11 perpendicular to the surface of substrate 13.
The distance that the microscope objective must be translated is equivalent to the thickness W1 of the substrate 13 divided by the index of refraction n of the substrate 13 (e.g., n=1.5). For instance, the microscope is first centered on the lower alignment mark 14, often with the aid of a reticle or cross hair in the eyepiece of the microscope. The microscope is then vertically translated to focus on the top or photoresist surface of the substrate, where the mask is moved laterally to center its alignment mark in the field of view of the microscope. After exposing and developing the photoresist, the substrate is etched to transfer the pattern from the photoresist into the surface of the substrate.
To achieve accurate top-to-bottom alignment using a conventional mask aligner, as described, the microscope must be precisely translated in a direction perpendicular to the surfaces of the substrate. If the microscope is not translated perpendicular to the surfaces, a lateral change in position of the microscope will result, causing the two patterns on the opposite surfaces to be misaligned.
Conventional mask aligners are not generally designed for precise perpendicular translation of the microscope body. Rather, the normal wobble and straightness of travel tolerances in mask aligner microscope translation stages is large enough to introduce several microns of lateral error in the alignment. In fact, recent experiments using a state-of-the-art conventional mask aligner showed more than twenty (20) microns of lateral alignment error between the patterns placed on opposite surfaces of a typical substrate. Consequently, conventional mask aligners of this type are susceptible to error.
Another conventional system used to achieve front-to-back alignment involves two video cameras used to focus upon the alignment marks positioned on opposite sides of the substrate, the two images from the cameras being superimposed electronically to show lateral alignment of the two marks. However, use of this system to align substrates of different thicknesses is limited, since the system must be calibrated for a fixed substrate thickness using a calibration plate which has alignment marks precisely placed on both sides of the plate by the manufacturer of the mask aligner.
The present invention is directed to an apparatus and method that substantially obviates one or more of the problems experienced due to the above and other limitations and disadvantages of the conventional art.
An object of the present invention is to provide a projection eyepiece and method for aligning pattern areas on opposing substrate surfaces with improved accuracy.
Other and further objects, features, and advantages of the present invention will be set forth in the description that follows, and in part will become apparent from the detailed description, or may be learned from the practice of this invention.
To achieve these and other objects, features, and advantages in accordance with the purpose of the present invention as embodied and broadly described, the present invention includes a protection eyepiece device that detects alignment between positions on opposing surfaces of a substrate having a reflective surface on at least a portion of one surface side, the eyepiece including a reticle source structured and arranged to project a reticle image toward the substrate, and a detection device structured and arranged to receive a reflection of the reticle image from the substrate and to determine alignment of the positions on opposing surfaces of the substrate based on the received reflection. The reticle source is structured and arranged to project the reticle image with a focal point on a first side of the substrate, the reflection of the reticle image generally being received by the detection device from a second and opposing surface of the substrate. The reticle source generally includes a source capable of projecting light and a reticle position to receive the projected light from the source, the reticle being structured and arranged such that light therefrom forms a reticle image. The detection device generally includes a plane upon which the received reflection is compared with at least one of the projected reticle image and a representation of the projected reticle image. A second reticle is sometimes used to generate the representation of the projected reticle image on the plane with which the received reflection is compared to determine alignment. The reticle source may further include a beam splitter on which the reticle image is instant, the beam splitter being structured and arranged to split the instant reticle image such that the reticle image is projected toward the substrate and toward the detection device. The reticle device may alternatively include a polarization sensitive beam splitter upon which the reticle image is incident, the beam splitter being structured and arranged to reflect instant light of a first predetermined polarity and pass instant light of a second predetermined polarity, and a linear polarizer positioned to receive light from the source, the linear polarizer being structured and arranged to pass light of the first predetermined polarity such that the light passing through the linear polarizer is reflected by the polarization sensitive beam splitter. The beam splitter is structured and arranged to reflect light of the first predetermined polarity toward the detection device. The detection device generally includes an image perceiving device structured and arranged to receive at least the reflection of the reticle image from the substrate and a second linear polarizer positioned between the reticle and the image perceiving device, the linear polarizer being structured and arranged to block light from the source that passes through the reticle. In the projection eyepiece, the reticle is a reflective reticle that generates the reticle image by reflecting light incident from the beam splitter, the reticle being positioned between the image perceiving device and the beam splitter. The reticle source generally includes a quarter wave plate positioned between the beam splitter and the reflective reticle, and the detection device includes a quarter wave plate positioned between the reflective reticle and the second linear polarizer.
When the source is structured and arranged to project polarized light, the reticle source generally includes a non-polarizing beam splitter upon which the polarized light is incident, the beam splitter being structured and arranged to reflect the incident polarized light and to pass light that is orthogonal to the incident polarized light. The detection device generally includes an image perceiving device structured and arranged to receive at least the reflection of the reticle image from the substrate, and a linear polarizer positioned between the reticle and the image perceiving device, the linear polarizer being structured and arranged to block light passing through the reticle from the source. The detection device may also include a quarter wave plate positioned between the beam splitter and the substrate.
The source generally includes a light source capable of generating light, a collimator capable of collimating the generated light, and a linear polarizer capable of polarizing the collimated light. The source may alternative include a light source capable of generating light, a focusing lens and diffuser grating capable of focusing the generated light onto reflected positions of the reticle and a linear polarizer capable of polarizing the collimated light.
The present invention also includes a method for detecting alignment between positions on opposing surfaces of a substrate having a reflected surface on at least a portion of one surface side.
Both the foregoing general description and following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Thus, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention are given by way of example only, since various changes and modifications that are within the spirit and scope of the invention will become apparent to those of ordinary skill in the art from this detailed description.